In aqueous systems, particularly industrial aqueous systems such as cooling water systems, boiler systems, and the like, it is undesirable to have contaminant deposits accumulating on the internal surfaces of the equipment and pipes and the like. Such deposits may interfere with water flow, accelerate corrosion of metal parts and surfaces, reduce heat transfer, and lead to other problems. Among the undesirable deposits that may form and accumulate in aqueous systems are deposits of insoluble iron compounds that are derived from soluble iron present in the water of the system.
Soluble iron may be introduced into an aqueous system by its presence in the feed or source water or in other ways. Soluble iron is generally believed to be iron in the form of its ferrous ion, which is in its second oxidation state, as Fe.sup.+2, generally referred to as "iron (II)". Soluble iron, as this term is used herein, does not exclude, however, other forms of iron that may be dissolved or soluble in a given water system.
In aqueous water systems, and in particular industrial aqueous water systems, the presence of soluble iron in the feed or source water, or the introduction of soluble iron species into the water of the system by other means, can lead to the undesirable precipitation of iron oxides and iron hydroxides which may settle and accumulate as deposits on surfaces in contact with the water. Such precipitation occurs when the waters containing such soluble iron are changed to a more oxidative environment. In other words, such soluble iron may precipitate from solution upon encountering oxidizing conditions. A very typical oxidizing condition encountered in industrial aqueous systems is a water pH of greater than 7.0. For instance, a typical source water has a pH of less than 7.0. When such source water is brought into an aqueous system, it may undergo an increase in pH, for instance by being admixed with water having a higher pH or being subjected to water treatment(s) that raise the pH. At least some oxygen is generally present in such water. The precipitation of iron oxides and hydroxides from waters initially holding soluble iron upon an increase in pH to greater than 7.0, and more typically upon an increase in pH to 8.0 or higher, some oxygen being present, is a common phenomenon. Such oxidizing conditions which can lead to such precipitation are not limited to increases in pH, but also include for instance the introduction of, or increase in the level of, an oxidizing agent such as chlorine and the like. Thus two factors are involved, i.e., the presence of soluble iron in waters introduced into an aqueous system, and then the in situ precipitation of iron oxide and hydroxide when the oxidizing condition of the aqueous environment is increased.
As to iron precipitates, it is believed that most of such precipitate is composed of ferric oxide and ferric hydroxide, wherein the iron is in its oxidation state of three, referred to as "iron (III)". In the field of the present invention and for the purposes of the present invention the relative proportions of the precipitate between ferric oxide and ferric hydroxide is not important nor does it matter whether some, or even all, of the iron precipitate is ferrous oxide or ferrous hydroxide. Hence the iron precipitate, which is generally a mixture of ferric oxide and ferric hydroxide, possibly with some ferrous species therein, will be referred to hereinafter as iron (hydr)oxide. It is also to be understood that the term iron hydr(oxide) includes any and all hydrated forms of such oxides and hydroxides.
In a typical aqueous system, the reduction or elimination of iron hydr(oxide) deposits is not the only problem addressed in treating the water to avoid the accumulation of contaminant deposits. For instance inhibition of scale deposits such as calcium phosphate scale, calcium organophosphate scale, and the like, is often important. The presence of iron has at times been found to interfere with chemical treatments employed to inhibit such types of scale. Hence a water treatment that reduces or eliminates iron hydr(oxide) deposits that also inhibits other scale deposits despite the presence of iron has additional advantages in water treatment as to efficacy and economy.
It is an object of the present invention to provide a process that reduces or eliminates iron hydr(oxide) deposits that are, or can be, formed when the oxidizing conditions of an aqueous system that contains soluble iron are increased. It is an object to provide such a process wherein the presence of iron does not interfere with at least some scale inhibition treatments, for instance treatments to inhibit calcium phosphate scale, calcium organophosphate scale, and like, including situations wherein the present process has the additional effect of inhibiting such scale. These and other objects of the invention are described in more detail below.